1. Field of the Invention
The present invention relates to an apparatus and method for converting two-dimensional (2D) line segment data that is input through a 2D input device such as a mouse or tablet to three-dimensional (3D) data to facilitate the construction of a 3D wireframe graphics. The invention further relates to an apparatus and method for displaying line segments of 2D data and line segments of 3D data in a mixed form on a 2D display device such as a CRT.
2. Description of the Related Art
Due to the absence of information in the depth direction, no currently available combination of a 2D input device and a 2D display device can provide direct graphic input of a 3D wireframe graphics to an apparatus.
For example, as shown in FIG. 17A, in the case of inputting a line segment GH as a 3D line segment with an endpoint G selected as the origin and displaying it on a 2D plane (in this case, the paper surface of the figure), the line segment GH can be specified as a 3D line segment having 3D data if the position of the other endpoint H is determined as 3D data. However, since the position of the endpoint H cannot be determined three-dimensionally due to the absence of information in the depth direction, the line segment GH cannot be specified three-dimensionally; that is, it may be the line segment shown in FIG. 17B or FIG. 17C, or one in another position. That is, a 3D graphics constructing apparatus cannot specify the 2D line segment GH as a 3D line segment by data processing.
When a designer sketches a 3D object on, for example, a paper surface using a writing instrument, he recognizes, using his own senses, the 3D position of a line segment like the above line segment GH (i.e., endpoint H) even if it has not been determined, and, as the sketch progresses, he establishes the position of a line segment that has not yet been specified three-dimensionally based on its relationship to later-drawn line segments. In this manner, he can draw a 3D object image consisting of line segments whose positions are three-dimensionally determined as a whole.
However, conventional data processing using a computer can produce a 3D wireframe graphics and display it on a 2D display screen only in the case where coordinate data for each of the line segments (i.e., its endpoints) that constitute the graphics are three-dimensionally specified. Therefore, a 3D graphics is input in a way such that the 3D coordinates of its vertices are directly input from a keyboard in the form of numerical values, or in a way such that using a three-plane diagram three views of the graphics are separately input in a graphic manner and thereafter corresponding points among the views are indicated by an operator.
In the former method, however, mistakes in order of magnitude are likely to occur. The latter method has the problem that error is likely to occur in indicating the corresponding points, and it is also cumbersome to draw a plurality of views of the same object as viewed from different directions.
There is known a method in which a 3D line segment to be drawn is input to an apparatus by inputting its coordinates along reference 3D coordinate axes (Agui et al., "Solid Object Input System by Conformal Projection Method," Transactions of Information Processing Society of Japan, January 1987, Vol. 28, No. 1). However, in this method, 3D coordinates are simply input in a graphic manner, and the line segment still cannot be drawn (i.e., displayed) without provision of all the 3D coordinates.
Another method is known in which the inputting is performed such that a plane is specified in a 3D space and made to have a one-to-one correspondence with coordinates of a 2D plane for the inputting (E. Sittas, "3D Design Reference Framework", Computer-Aided Design, 1991, pp. 380-384). However, this method is associated with the problem of a cumbersome input operation because a plane must be defined before each input of a line segment.